This invention relates to the art of measurement for detecting the state of motion of an internal moving medium of a living body, and more particularly to a novel technique which is effectively, applicable to an ultrasonic diagnosis apparatus used for measurement of the moving speed (referred to hereinafter simply as speed) of an internal moving medium of a living body or measurement of the moving speed and moving speed dispersion (referred to hereinafter simply as speed dispersion) of such a moving medium.
An ultrasonic pulse-Doppler method has been put into practical use for the measurement of the speed of an internal moving medium of a living body, for example, a visceral organ such as the heart or a fluid such as blood or humor in a circulatory organ.
A prior art, ultrasonic diagnosis apparatus utilizing the ultrasonic pulse-Doppler method described above is disclosed in, for example, Jpaanese Unexamined Patent Publication No. 58-188433 (JP-A-58-188433) laid open on Nov. 2, 1983. In the cited patent publication, an autocorrelator is employed for converting a high-frequency signal reflected from an internal moving medium of a living body into complex signals and then computing autocorrelation between the complex signals, and a speed operator is employed to compute the speed of the moving member on the basis of the computed autocorrelation. However, the necessity for incorporation of the autocorrelator has required a complicated circuit structure resulting in a bulky overall size of the ultrasonic diagnosis apparatus.
In the prior art, ultrasonic diagnosis apparatus disclosed in the cited publication, delay line cancellers are employed together with the autocorrelator for detecting the speed of the internal moving medium of the living body on the basis of the high-frequency signal received by an ultrasonic probe and including a Doppler shift frequency. Numerical formulas used for carrying out a series of signal processing on the received high-frequency signal by the delay line cancellers and autocorrelator do not include amplitude terms, and the speed of the moving medium is computed under the assumption that the amplitude of the received high-frequency signal is constant. Consider now the case of detection of the speed of blood flow, as an example. In this case, it is limitatively assumed that the second power of the amplitude (energy) of the Doppler shift signal is proportional to the number of blood cells scanned by the ultrasonic beam, and the number of blood cells scanned by the ultrasonic beam is constant regardless of the rate of blood flow.
However, the number of blood cells scanned by the ultrasonic beam is not always constant regardless of the rate of blood flow. This is because turbulence may occur in the blood flow, and there is a concentration difference between red blood cells and white blood cells. Thus, the amplitude of the Doppler shift signal is not always constant. Further, although the power of the ultrasonic signal attenuates while the ultrasonic signal is received after it is transmitted into the living body, the amount of beam attenuation varies depending on the tissue structure of the scanned portion of the living body. Therefore, in the case of a moving medium such as the heart where the momentum is large, the state of the tissue structure, through which the ultrasonic beam passes, varies continuously, and it is unable to always receive a high-frequency signal including a Doppler shift signal having a constant amplitude.
Thus, the prior art manner of signal processing, according to which the average speed is computed under the assumption the amplitude of the received signal is always constant, has been defective in that an error is inevitably included in the result of computation.